Off-set screen printing form thick-film prints comprising ink, paste, or the like on the surface of a substrate using a printing plate (screen mesh); one such type of off-set screen printing is the so-called silk screen printing, which is used to form fine patterns at high production rates; hence, it is utilized in a wide variety of industrial fields.
In the field of electronic parts production, screen printing method has been employed from the points of both precision and mass-production. In this field, the demand for forming finer print patterns with high precision has steadily increased due to recent development of technology to miniaturize the sizes of electronic parts.
LTCC is now a popular technology for manufacturing high-frequency circuits and is used advantageously to print 3-D circuits within a ceramic block by enabling integration of passive elements such as resistors, inductors and capacitors with fine conductor patterns. This LTCC approach also allows a number of interfaces and the reduction of the overall substrate size. LTCC technology utilizes highly conductive metal and has a low dielectric constant, low surface roughness, low sintering temperature, and good thermal properties.
Standard screen printing technology has also been principally developed for hybrid circuit manufacturing. Hybrid circuits are electronic modules printed on ceramic substrates, a technology in between semiconductor integration and discrete realization on PCB technology, and they are commonly used when electronic modules have to meet high technical requirements. The advantages of screen printing technology are well known: versatility in the conception, miniaturization, and mass production at low cost. The thick film components are produced by screen printing of conductive, resistive, and dielectric layers in order to form passive components on an LTCC substrate. Fine line printing is used to achieve high-component density. Therefore, it is important to control each screen printing parameter to improve on the quality of components and the yield of the circuit.
In general, screen printing is the basic technology for thick-film micro-circuitry. Many variables will affect the screen printing process. For example, the setting of the screen printer is a manual operation, and the quality of screen printed thick-film strongly depends on the operator and the process variables. The parameter settings affect directly the desired thickness and uniformity of the pastes printed on the substrates.
U.S. Pat. No. 6,945,167, assigned to Matsushita Electric, discloses a screen printing apparatus and method. It discloses that the print parameter settings include a squeegee movement speed, a printing pressure, and plate release conditions. The squeegee movement speed is set at the first step, then a printing pressure for realizing a desired cream solder charging state is set at the second step, and then plate release conditions for realizing a desired cream solder transfer state is set at the third step. It does not mention firing, baking, or heating of ceramic printed board, and there is no mention of LTCC or green tape.
U.S. Pat. No. 4,817,524, assigned to Boeing, discloses a method for screen printing, drawing a contact edge of a squeegee on the screen in a feed stroke such that a layer of paste is deposited on the screen, and then drawing the squeegee over the screen in a print stroke with the contact edge in contact with the screen so that the paste is forced through the screen onto the substrate. It does not mention LTCC or ceramic printed board.
U.S. Pat. No. 5,699,733, assigned to the Industrial Technology Research Institute, discloses a process that requires firing at low temperature, i.e., 500-600° C. However, the process is directed to increase paste layer thickness by subsequent repeated layering up to 6 layers. There is no mention of 3-D circuit or interconnecting circuit layers as required in an LTCC process.
U.S. Pat. No. 5,448,948, assigned to Delco Electronics Corp, discloses a screen printing device for screen printing a thick-film paste through a screen so as to form a substantially void-free film on a surface of a microelectronics circuit. It is limited to squeegee design.
U.S. Pat. No. 4,604,298, assigned to Gulton Industries, Inc, is directed to the viscosity of conductive paste compound, a high-viscosity gold alloy, firing at 800-900° C. However, there is no mention of ceramics and no mention of 3-D circuitry or embedding of components.
U.S. Pat. No. 7,930,974, assigned to Mitsubishi Electric Corp, discloses vacuum suction holes for affixing a substrate to be printed. There is no mention of green tape or ceramic being made. Baking is disclosed for electrode material to form electrodes.
U.S. Pat. No. 7,908,964, assigned to Panasonic Corp, discloses specifically to the clearance gap between the screen mask and substrate. There is no mention of ceramic firing, baking, or application for LTCC green tape.
Chinese Patent No. 101188260, issued to Shanghai Univ., et al, discloses an LTCC process for fabricating a square or circular cavity as a base for a high-powered LED to be formed on the LTCC layer prior to screen printing. There is no mention of any process control parameters for screen printing.
Chinese Patent No. 101777413, issued to Shenzen Sunlord Electronics, discloses a process for forming an LTCC power inductor comprising ferrite magnetic core; it mentions the advantages of high-frequency ceramic material and thinner (finer or higher resolution) screen print lines besides other benefits such as less conductor loss, low dielectric constant, better coefficient of heat conductivity and better exothermic property. The process control parameters disclosed here are applicable for a very specific type of device, i.e., for an LTCC power inductor.
It can thus be seen that there exists a need for improving the processes for forming co-planar waveguides using LTCC substrates.